The present invention relates in general to electronic circuits and, more particularly, to a circuit for controlling an effective resistance between two nodes.
Most if not all electronic circuits commonly use resistors to set the current flow between two nodes. Fixed resistors can easily be implanted into an integrated circuit, or otherwise placed in a discrete circuit, to set a particular resistance value. The current flow is determined by the well known Ohm's law where I=V/R.
Many applications such as filters require tuning or adjustments to achieve the intended circuit function. For example, the cut-off frequency of a low-pass filter is set by selecting particular values of resistors and capacitors. In the prior art, the resistor value is often set during calibration and test by a trimming operation where sections of the resistor are removed until the circuit operates as desired. However, once the resistor value is fixed, it becomes difficult to change. Any change in conditions such as temperature, moisture, and component drift can effect the circuit operation.
One potential solution is to use variable resistors that can be adjusted with changing conditions. Resistors need to be bi-directional in order to behave properly with AC signals. That is, the resistor must be able to float with high impedance at both ends. Transconductance amplifiers are generally uni-directional devices, i.e. floating at one end with the other end at a low impedance voltage source, and therefore not well suited for bi-directional operation. The prior art has shown that it is possible to achieve bi-directional operation with certain configurations of transconductance amplifiers. For example, a three-amplifier arrangement that provides bi-directional variable resistance is shown in a book entitled Introduction to the Design of Transconductor Capacitor Filters by Jaime E. Kardontchik, Kluwer Academic Publishers, page 27. However, the three-amplifier arrangement serves as only a single variable resistor. In applications that require several variable resistors, such an approach is impractical as it becomes difficult to control all three amplifiers simultaneously and the complex circuit consumes an excessive amount of physical space.
Hence, a need exists for a simple, controllable resistance circuit that can operate either uni-directional or bi-directional.